The mouse embryoid body/teratoma system OTT 6050 developed by Leroy Stevens appears to be an ideal system for studying the molecular mechanism of cellular differentiation in a mammalian system. Thus the following studies were carried out. Using oligo(dT)-cellulose chromatography, total poly(A)-containing RNA was isolated from simple embryoid bodies and tumors derived from the embryoid bodies. These RNA samples were characterized with respect to poly(A) content, size on denaturing sucrose gradients, and poly(A) tract length. They also were tested for translation activity in an in vitro lysate system. Complementary DNA was synthesized using reverse transcriptase for both the embryoid body and the teratoma poly(A)-RNA preparations by the RNA-complementary DNA hybridization technique (RNA excess). An evaluation of the sequence complexity of the preparations could be made by computer analysis of the hybridization curves. Three classes of poly(A)-containing RNA could be resolved in both embryoid bodies and teratomas based on their relative abundance in the respective RNA preparations.